1. Field of the Invention
The present invention relates generally to the area of shaping a parison in a blow molding process, and more particularly, to a die for extruding a tubular parison having a nonuniform circumferential wall thickness.
2. Description of the Related Art
Polyolefins, such as polyethylene, polypropylene and copolymers of ethylene and propylene are commonly used in blow molding processes to produce containers such as bottles. In a typical blow molding process, a tubular parison, or container preform, is produced by extruding a thermoplastic material vertically downwardly through a circular die having a generally cylindrical flow channel which diverges or converges in diameter over a short flow channel segment terminating at the die outlet. Therefore, that short flow channel segment is a frustoconical channel bounding a volume shaped like either a frustum or an inverted frustum. The circular die includes a die ring and a mating mandrel located within the die ring. Typically, the mandrel is mounted so that it moves longitudinally with respect to the die ring under manual or automatic control. Relative motion between the die ring and mandrel varies the die gap at the die outlet to control the wall thickness of the tubular parison. The vertical, molten, tubular parison drops between two halves of an open mold, and the mold closes squeezing the top and bottom ends of the parison together. That squeezing action changes the generally circular cross-section and shape of the tubular parison to a generally elliptical cross-section and shape. A gas, such as air, is injected through one end of the mold into the interior of the parison and pushes the molten tubular parison against the internal surfaces of the mold. After a cooling period, the mold is opened, and the blow molded article, such as a container or bottle, is ejected therefrom.
When the molten parison changes from a generally circular to a generally elliptical shape, the spatial relationship between the parison's tubular side walls and the internal surfaces of the mold changes significantly. Consequently, in those situations where the shape of the desired container does not correspond to the elliptical shape of the parison, during the blowing process, the tubular side walls of the elliptical parison are stretched over different lengths. Further, the thickness of the parison wall is inversely proportional to the length over which it is stretched. Therefore, a parison with an elliptical cross-section will produce a container having a varying cross-sectional wall thickness profile, that is, the wall thickness of the container as measured around a cross-sectional perimeter will vary. Nonuniformity of wall thickness produced by changes in the spatial relationship between the parison and the internal mold surfaces is a problem in manufacturing a container of any shape. However, it is often more of a problem when producing noncircular containers having triangular, rectangular or other multilateral shapes. Such shapes often have greater variations in the distances between the elliptical parison and the internal surfaces of the mold.
In blow molding multilateral bottles, such as, square bottles, the mold is typically constructed to have the mold parting line bisect the mold diagonally across a first pair of mold corners. Therefore, when the mold is closed squeezing the parison into an elliptical shape, the major axis of the elliptical shape of the parison aligns with the mold parting line. The parison side walls at the ends of the major axis of the ellipse are closer to the first pair of mold corners along the mold parting line than they would be if the parison were circular. The second pair of mold corners, perpendicular to the mold parting line, are coincident with the minor axis of the elliptical parison. The parison side walls at the ends of the minor axis of the elliptical parison shape are further from the second pair of mold corners than they would be if the parison were circular. Consequently, when using a standard circular extrusion die to produce a molten tubular parison having a constant wall thickness, the wall thickness of the blown bottle in the second pair of corners perpendicular to the parting line stretches more within the mold and has less material than the other peripheral wall sections including the first pair of corners. Therefore, the strength of the bottle, as measured by its resistance to column crush, that is, resistance to crush from a force applied along its longitudinal axis, is significantly reduced.
To overcome the above disadvantage of producing a multilateral bottle with a nonuniform wall thickness, it is known to use parison extrusion dies having circumferential ovalization grooves or segments located opposite the mold corners perpendicular to the mold parting line to increase the die gap across those segments. For example, as shown in U.S. Pat. No. 3,309,443, issued on Mar. 14, 1967 to J. Scott et al., the ovalization segments produce a parison having a nonuniform wall thickness in which the parison wall is thicker at the locations corresponding to the bottle corners perpendicular to the mold parting line. A typical ovalization segment produces an expanded die gap opening with a circumferential width in a range of from 60 degrees to 90 degrees. The depth of the ovalization segment is the radial expansion of the die gap and is measured from the nonovalized die wall. The depth of ovalization is in a range of from 0.001 inches (0.025 mm) to 0.010 inches (0.25 mm). The depth varies from zero at the ends of the circumferential width of the ovalization segment to its full depth value at the midpoint of the circumferential width of the ovalization segment. The ovalization length extends up the frustoconical segment, or land, of the die ring or mandrel in a circular arc. The ends of the arc coincide with the ends of the width of the ovalization segment; and the maximum length, which is the maximum radial length of the arc, occurs at the midpoint of the circumferential width of the ovalization segment. Typically, the ovalization length is defined as a percentage of the maximum radial length of the ovalization arc to the total length of the frustoconical segment in the die.
The ovalization segment at the die outlet increases the die gap within the ovalization segment and in turn, increases the mass flowrate or volume flowrate of material through the ovalization segment at the die outlet, thereby providing a thicker parison wall downstream of the ovalization segment. The thicker parison wall provides more material to flow into those areas of the mold where the parison is stretched over a greater length. However, a problem with the above downstream ovalization segments is that the linear velocity of the material at different points across the circumferential width of the ovalization segment is nonuniform and varies substantially from the linear velocity of the material in the nonovalized areas of the die gap. The linear velocity of the material at the midpoint of the ovalization segment may be 50% more than the linear velocity in the nonovalized sections of the die gap. Therefore, material downstream of the ovalization segment is flowing faster than adjacent material downstream of the nonovalized die gap areas. Significant linear velocity variations at different circumferential points of the parison wall are manifested by the material in the thicker heavier parison wall sections flowing over itself, that is, rippling, as it flows down the length of the parison.
That velocity difference plus gravitational forces also cause the thicker and heavier portions of the parison walls to move inwardly and the thinner wall sections of the parison to move outwardly thereby creating a parison having a generally elliptical cross-section. That generally elliptical shape is produced before the parison is clamped by the mold. The tendency for the thicker molten wall section to come together increases as the parison length increases. In the most extreme case, the parison deformation can quickly reach the point where the thicker heavier wall sections contact each other within the length of the mold, at which point, the parison is defined as collapsing. The collapsed parison cannot be blown; and consequently, the process must be stopped; the collapsed parison removed from the machine and the machine restarted. Even if the parison does not collapse, these variations in parison wall thickness can produce folds in the parison which result in undesirable defects in the bottom of the blown bottle.
To overcome problems caused by nonuniform velocities in a blow molding extrusion die with ovalization segments, U.S. Pat. No. 4,496,301 issued on Jan. 29, 1985 to L. Mozer et al., provides a set of second depressions at the upstream end of the frustoconical segment of the die which are equally spaced between the ovalization segments. The second, upstream depressions increase the velocity of the material flowing in the circular areas of the die between the downstream ovalization segments and attempt to reduce the velocity differentials created by the ovalization segments. By reducing velocity differentials, the rippling effect and the probability of parison collapse is reduced.
The above die designs change volume flow around the circumference of the parison to provide a material distribution that results in uniform wall thickness of a blown bottle. In all cases of prior die designs, changes in circumferential material volume flow have the disadvantage of producing variations in the linear velocity of the material at different circumferential points.